One technology for after-treatment of engine exhaust utilizes selective catalytic reduction (SCR), which facilitate certain chemical reactions to occur between NOx in the exhaust and ammonia (NH3). NH3 is introduced into an engine exhaust system upstream of an SCR device by injecting reducing agent (e.g., urea) into an exhaust pathway, or is generated in an upstream catalyst. Urea is one example of a reducing agent, where the urea entropically decomposes to NH3 under high temperature conditions. The SCR facilitates the reaction between NH3 and NOx to convert NOx into nitrogen (N2) and water (H2O). However, as recognized by the inventors herein, issues with reactivity arise during cold-starts (e.g., engine temperature less than an ambient temperature), where an SCR may not reach a temperature suitable for reacting with NOx.
To account for cold-start emissions, an exhaust system may include a first, compact SCR adjacent or closer to an exhaust manifold and a second SCR at a location downstream of the first SCR relative to a direction of exhaust flow. By doing this, the first SCR may reach a light-off temperature quickly, even during cold-starts, while the second SCR, larger than the first SCR, may treat emissions outside of engine cold-starts. However, such systems may be expensive and inefficient. As an example, multi-SCR systems may include separate urea injectors for supplying urea to the first and second SCRs. This may involve a more convoluted control system to operate the injectors. As another example, multi-SCR systems may include a single urea injector upstream of the first SCR, where the urea injector inundates the first SCR and allows exhaust gas to flow excess NH3 to the second SCR. However, this may be inefficient as the excess urea at the first SCR is consumed during higher engine loads (e.g., high load).
Other attempts to address multi-SCR systems include redirecting a urea injection via a bypass. One example approach is shown by Hirota et al. in U.S. Pat. No. 6,192,675. Therein, a bypass redirects a portion of exhaust gas mixed with urea to a second SCR downstream of the first SCR without flowing through the first SCR. Furthermore, the first SCR may comprise capillaries and/or other flow passages comprising no catalytic components such that urea passes therethrough without interacting with the first SCR.
However, the inventors herein have recognized potential issues with such systems. As one example, a flow control valve and appropriate valve actuator are located in the bypass, thereby increasing a manufacturing cost of the exhaust system and introducing components susceptible to degradation. Furthermore, the bypass passage introduces packaging restraints to the exhaust system, increasing its size and resulting in added weight to the system.
In one example, the issues described above may be addressed by a method for treating exhaust gases comprising adjusting a pressure of a reductant injector positioned upstream of a first catalyst in an exhaust passage, wherein the pressure alters a reductant distribution in the exhaust passage, responsive to an SCR temperature, wherein a second SCR device is arranged downstream of a first SCR device. In this way, reductant is sufficiently supplied to two aftertreament devices arranged in series along a passage without a bypass and corresponding bypass valves.
As one example, the first SCR device further comprises one or more flow-through regions which allow the passage of reductant from the injector to the second SCR device. The flow-through regions comprise a decreased catalytic composition compared to catalytic regions of the first SCR device. As such, exhaust gas carrying reductant flowing through the flow-through regions deposits little to no reductant. An amount of reductant flowing through the flow-through regions is adjusted by a reductant injection pressure, which may adjust a reductant radial distribution. In one example, the flow-through regions are located on an outer region of the first SCR and the catalytic regions are located along a central core, as such, an increased reductant pressure directs reductant toward outer regions of the first SCR device. Thus, a decreased reductant pressure, relative to the increased reductant pressure, directs reductant toward catalytic regions (e.g., the central core) of the first SCR device. Thus, the flow-through regions are strategically located along the first SCR device. To further decrease the cost of the present disclosure compared to that of Hirota, the first SCR device may be smaller than the second SCR device. As such, a lesser amount of precious metals may be included in the present disclosure compared to an amount used by Hirota.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.